Fuse for incandescent lamp



Nov. 4, 1958 s. M. GRAY EI'AL 2,859,381

FUSE FOR INCANDESCENT LAMP Filed March 20, 1956 IN VEN T 0R8: .swun M may J0/1W a. rmow: B RUBERT 5 REED ATTORNEY nit 2,859,381 Patented Nov. 4, 1958 .iee

FUSE roRmcANnnscnNr LAMP Samuel M. Gray, Marblehe'ad, and John O. Fellows and Robert F. Reed, Salem, -Mass., assignors to Sylvania Electric Products'lncorporated, Saiem, Mass, a cor poration of Massachusetts ApplicationMarch 20, 1955, Serial No. 572,670

' 2 Claims. ci. 315-74) This invention relates to fuses forming an integral part of incandescent lamps and to supporting structures for such fuses.

Because the filament of an energized incandescent lamp is sufiiciently heatedto emitelectrons, it is possible that an arc may form at the filament when a break occurs therein. The intense heat of such an arc, which starts at the break and progressively shorts out greater portions of the filament, may readily cause the lamp bulb to shatter unless a fuse or other current limiting device is incorporated in the lamps circuit. Accordingly, a fuse is conventionally provided as *an' integral part of an inca'ndescent lamp. Such -fuses,-which' are in series with the filament,'are generally disposed outside the bulb within the lamp base where they may advantageously be embedded in basing cement. In the event that an are develops at the fuse when the latter is melted by an overload, the basing cement serves to quench or snuff out the arc.

While the above-described externally mounted fuses are entirely satisfactory from the point of view of safety and well suited to lamps having screw or bayonet bases, they are not adaptable to many forms of lamps having other types of bases. Our invention is accordingly directed to the problems of providing fuses and supporting structures suited to being mounted within the lamp envelope.

An expedient which has heretofore been adopted to provide an internal fuse comprises a length of fusible wire mounted in one of the filament supports and a glass tube loosely encompassing the fusible wire. If an are develops at the fuse when the latter is blown by an overload, the tube by limiting the path of the arc to a small cross sectional area serves to quench the arc. Such an arrangement presents serious disadvantages. One of these is that the filament supporting structure is greatly weakened by the inclusion of the fuse wire. As a result, a break may occur in the fused supporting wire. Additionally, lack of suificient rigidity in the support permits excessive flexing of the filament by random forces which may also cause the filament to be distorted or to be broken I prematurely. The tube which forms a part of such an arrangement is also a disadvantage. Not only does the inclusion of the tube complicate assembly but its tendency to rattle audibly is objectionable in such applications as projection lamps for sound motion picture projectors.

I it is accordingly an object of our invention to provide an improved internally mounted fuse for incandescent lamps.

Another object is to provide a mounting structure for such a fuse which permits the filament to be adequately supported.

Still another object is to provide an internally mounted fuse which will of itself prevent the formation of a second arc at the fuse and thus allows additional arc-quenching means to be eliminated.

These and other objects are achieved in an illustrative embodiment of our invention by a fuse whose mounting is relatively independent of filament supporting members. The filament is mounted on a pair of support wires eX- tending within the envelope. One of the support wires, connected to one end of the filament, has an outwardly extending portion which forms one of the lamps terminals. The other support wire electrically connected to the other end of the filament need not extend through the envelope since We provide an additional terminal pin to form an electrical connection with a suitable socket. The fuse is provided with a relatively rigid mounting extension 'at each end, one of said extensions being fixedly connected to the second-mentioned filament support and the other to an internally extending portion of the terminal pin. The support, the terminal pin and the extensions have sufficient current-carrying capacity that they are not heated sufiiciently to emit electrons by an overload current which causes the fuse to melt. In addition, the gap left between the two extensions by the melted fuse is greater than the maximum arcing length of the voltage applied to the lamp.

The foregoingand other objects, advantages and novel features of our invention will be clear from the following description of an illustrativeembodiment taken in connection with the accompanying drawings in which:

Figure l is'a-view in 'front elevation of a projection lamp including a fuse according to our invention;

Figure 2 is a view in cross-section of the' lamp taken along the line 2-2 of Fig. 1;- I

Figure 3 is a view in cross section similar to Fig. 2 but showing an alternate fuse mounting.

Referring now to the drawings, a projection lamp in-. corporating a fuse according to our invention will be described in detail. The lamp includes a filament 12 enclosed Within a glass envelope or bulb 14. One end of the filament 12 is connected to support wire 16 by means of a filament insert 18 and the other end is similarly connected to support wire 20. The filament 12 is supported at intermediate points by wires 22 extending from glass bridges 24 above and below the filament. The bridges 24 are in turn fixed upon the support wires 16, 20.

The support wire 16 passes through the glass at the lower end of the envelope and its outer end provides an electrical terminal 28. On the other hand, although the wire 29, as shown in Figs. 1 and 2 may also pass through the bulb 14 in the same manner, electrical communication with its related end of the filament 12; is provided through pin 3% which also passes through the bulb 14 and is connected to an intermediate portion of the wire 20 through a fuse structure indicated at 32. The lamp is installed for use in a suitable socket having contacts corresponding to terminal 28 and pin 30 connected to an electrical power source. Base 34 is secured to the envelope 14 and includes a centralizing projection 36 having an orienting key not shown which enters a keyway in the socket to provide angular positioning of the filament 12 and insertion of the lamps electrical terminals into appropriate socket contacts.

We will now describe in greater detail the fuse structure 32 which comprises a fusible central element 38 to which a relatively rigid extension 40 is welded or otherwise secured at each end. The fuse structure 32 is rigidly supported by the extensions 40 which are fixed, one to the wire 20 and the other to the pin 30. As shown in Fig. 2, the fusible element 38 is disposed at a distance from the interior surface of the envelope. It has been found that such spacing greatly reduces'the possibility of injury to the glass from fuse fragments when the element 38 is melted by an overload.

Greater spacing may be provided by an arrangement shown in Fig. 3, in which parts of the lamp with the exception of a fuse structure indicated at 42 are identified by the same reference numerals as in the other views. The fuse structure 42 consists of a fusible element 44, a short mounting extension 46 connecting one end of the element 44 to the pin 30 and a second extension 48 at the other end of the element 44. The extension 48 is supported on pin 50 and extends to the support wire 20, forming an electrical connection. The fusible element 44 is thus centrally mounted in a transverse plane of the bulb 14. In both arrangements power is supplied to the lamp by connections with the pin 30 and terminal 28 which is the outward extension of the wire 16.

Since the operating characteristics of the two arrangements, which we have described, are essentially alike, the following description of the mode of operation will be restricted for clarity to the embodiment particularly shown and described in connection with Figs. 1 and 2. It will be appreciated however that whatever is said of the fuse structure 32 applies equally to the fuse structure 42. I

When a break occurs in the filament 12, an arc generally develops at the break and proceeds to short out progressively greater portions of the filament, thus reducing the internal resistance of the lamp. The element 38 has a current carrying capacity which is only slightly in excess of the normal filament current. As the current is increased by such an arc, the element 38 melts thereby interrupting current flow. The length of the element 38 is chosen to provide a gap between the inner ends of the extensions 49 which is too great to support an arc in the lamps atmosphere. Since the fuse structure 32 is relatively rigid and the extensions 40 fixedly mounted,

danger of a gap sufficiently narrow to permit a second arc to develop between adjacent ends of the extensions is eliminated. In addition, the fuse supporting structure is not heated to electron-emitting temperature by current suficient to melt the fusible element 38.

What we claim is:

1. An incandescent lamp for operation at a predetermined voltage, said lamp comprising a glass envelope, a metal filament therein, metal support wires for said filament, each of said support Wires being sealed into said envelope and at least one of said wires being sealed through said envelope to act as an external contact, a metal pin sealed through said envelope to act as a second external contact, said pin being spaced from said support wires a distance greater than the maximum arcing distance for said predetermined voltage, and a fuse wire inside said envelope and connected between said pin and one of the support wires for said filament, the other support wire for said filament being one sealed through said envelope to act as an external contact therefor.

2. The incandescent lamp of claim 1, in which the filament is at the middle of the glass envelope, and the fuse at an end of the envelope.

References Cited in the file of this patent UNITED STATES PATENTS 2,147,584 Thomas Feb. 14, 1939 2,179,455 Spaeth Nov. 7, 1939 2,183,952 Anderson Dec. 19, 1939 

